OXYGENATION OF (3Z)-ALKENAL TO (2E)-4-HYDROXY-2-ALKENAL IN SOYBEAN SEED (GLYCINE-MAX L)

(3Z)Alkenals, such as (3Z)-hexenal and (3Z)-nonenal, are produced from polyunsaturated fatty acids via lipoxygenase and hydroperoxide lyase catalysis, but in soybeans (Glycine max L.) (3Z)-alkenals have a fleet ing existence. In this study it was shown that soybean seeds possess t wo pathways that metabolize (3Z)-alkenals. One is a soluble (3Z):(2E)- enal isomerase that transformed (3Z)-hexenal and (3Z)-nonenal into the corresponding (2E)-alkenals. The other was a membrane-bound system th at converted (3Z)-hexenal and (3Z)-nonenal into (2E)-4-hydroxy-2-hexen al and (2E)-4-hydroxy-2-nonenal, respectively. The latter conversion w as shown to absorb O-2 with a pH optimum of 9.5. Little effect observe d with lipoxygenase inhibitors suggested that oxidation was not cataly zed by lipoxygenase. Instead, a specific (3Z)-alkenal oxygenase was im plicated in forming intermediate alkenal hydroperoxides. Hydroperoxide -dependent peroxygenase (epoxygenase) is known to reduce hydroperoxide s to their corresponding hydroxides and is also known to be inhibited by hydrogen peroxide preincubation. Consequently, intermediate 4-hydro peroxy-2-alkenals could be observed after inhibiting hydroperoxide-dep endent peroxygenase by preincubation with hydrogen peroxide. Because 4 -hydroxy-2-alkenals are potent toxins, these compounds may be produced as nonvolatile plant defensive substances.